WHATS WRONG WITH THE RELAY,WIDE PATH? 31 March 2005
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>Please explain how RELAY causes DUPES in the USA?
Thanks for asking, because it is important that eveyone understand
this and it is why RELAY in our current overloaded nets is now part
of the problem and not the solution...
The problem is that in the USA, Kantronics implemented my WIDEn-N
suggestions in 1998 over the top of the previous legacy RELAY-WIDE
system independently. A packet digipeated with RELAY, or WIDE in
the path is not compared with a subsequent WIDEn-N copy of the same
packet. Thus dupes can be generated. Lets assume a mobile is using
the path of RELAY,WIDE:
If all he hits is a rare RELAY-only digi in the boonies, then there
is one copy digipeated and then that copy is then digipeated by lets
say 3 other WIDES. Those are all 4 copies of the original packet,
one for each digipeater. We dont call those dupes. Those are what
we want. 4 copies, one for each of the 4 surrounding digis. That's
good.
MOBILE>APRS,RELAY*,WIDE
MOBILE>APRS,RELAY,DIGI1*
MOBILE>APRS,RELAY,DIGI2*
MOBILE>APRS,RELAY,DIGI3*
But all the other 95% of the time while the mobile is driving around
town his RELAY,WIDE packet is being heard by lets say 3 other digis.
Now then here is what we see:
MOBILE>APRS,DIGI1*,WIDE
MOBILE>APRS,DIGI2*,WIDE
MOBILE>APRS,DIGI3*,WIDE
MOBILE>APRS,DIGI1,DIGI2*
MOBILE>APRS,DIGI1,DIGI3*
MOBILE>APRS,DIGI2,DIGI1*
MOBILE>APRS,DIGI2,DIGI3*
MOBILE>APRS,DIGI3,DIGI1*
MOBILE>APRS,DIGI3,DIGI2*
for a total of 9 packets. Three are the intended packets from the 3
surrounding digis and the other SIX are "DUPES". Each digipeater is
digipeating the same packet TWICE because it does not compare the
RELAY digipeated packet to any that it has also digipeated vis WIDEn-N.
Now imagine each time that mobile passes by a HOME RELAY, that adds
another original for each of the digis to also digi a 3rd time (total
of 12 packets).
Not only is this so obvious to see happening and so obviously a waste
of bandwidth by causing a factor of 3 dupes on every packet transmitted,
but the stations doing it the most are MOBILES which already generate
10 to 20 times as many positions per hour as any other stations.
It is this totally unneeded multiplication of DUPES from RELAY,WIDE
paths that is the biggest load on the APRS channel in the USAand also
the *easiest* to get rid of.
Some people say that they have to have a RELAY in a certain area because
it is a dead zone. True, but the "cost" of having mobiles use the
RELAY,WIDE path just for that one packet in the morning and that one
packet in the evening in that "hole" is causing the other 40 of his
commute packets to be generating over 240 "DUPES" all the rest of the
time when he is closer to town on his 20 minute commute. We must
get rid of RELAY (and WIDE) which have no dupe elimination algorithm.
Besides, we can still have FILL-IN digis by just swapping WIDE1-1 in
place of the RELAY alias.
To me, we are shooting ourselves in the foot with all these mobiles
driving around using a RELAY,WIDE path. Yes, I advocated that for
years for backwards compatiblity with all the RELAY-only and WIDE only
TNC's we had back here on the east coast. But it is time to move beyond
that. In fact, we should have done it years ago.
But I have to admit that since I have been spending 12 hours a day for
the last 4 years building satellites, I had not been watching what was
going on on the local 144.39. A few months ago, I had a weekend lull
and was watching RAW packets and was *astonished* at what I saw from
all the dupes of all the RELAY, and WIDE paths in our area.
By the way, WIDE,WIDE does exactly the same thing. If there are 3 digis
that hear that path, then there are the potential for 9 copies just from
the first 3 digis. We MUST phase out WIDE completely and discourage
any routine use of RELAY.
On the other hand the path of WIDE2-2 goes just as far, but because the
WIDEn-N algorithm has perfect dupe elimination, then if there are 3 digis,
then there are only 3 copies (no dupes) heard in the original area of
packet origination. Copies only radiate outward and never fold back.
Whereas with RELAY,WIDE and WIDE,WIDE paths there will always be dupe
multiplicatiion.
But you ask, what about callsign-substitution? Isnt that supposed to
eliminate dupes and keep a digi from digipeating something it has
digipeated before? Yes, but look back at the original 6 dupes in the
original example above. Notice that none of the 6 dupes was ever
transmitted by the same digi twice. Those would look like
MOBILE>APRS,DIGI1,DIGI2,DIGI1* and you dont see those, beause substitution
prevents it. But that only protects after the 2nd hop. Most dupes are
caused after the first hop! There, could be as many as 15 copies
(3 desired and 12 dupes) heard from these 3 digis when only 3 would be
heard if WIDEn-N were used.
MOBILE>APRS,DIGI1*,WIDE,WIDE
MOBILE>APRS,DIGI2*,WIDE,WIDE
MOBILE>APRS,DIGI3*,WIDE,WIDE
MOBILE>APRS,DIGI1,DIGI2*,WIDE
MOBILE>APRS,DIGI1,DIGI3*,WIDE
MOBILE>APRS,DIGI2,DIGI1*,WIDE
MOBILE>APRS,DIGI2,DIGI3*,WIDE
MOBILE>APRS,DIGI3,DIGI1*,WIDE
MOBILE>APRS,DIGI3,DIGI2*,WIDE
MOBILE>APRS,DIGI1,DIGI2,DIGI3*
MOBILE>APRS,DIGI1,DIGI3,DIGI2*
MOBILE>APRS,DIGI2,DIGI1,DIGI3*
MOBILE>APRS,DIGI2,DIGI3,DIGI1*
MOBILE>APRS,DIGI3,DIGI1,DIGI2*
MOBILE>APRS,DIGI3,DIGI2,DIGI1*
That is why I say that any use of RELAY or WIDE paths is causing 2 to
5 times the duplication of every packet transmitted in the USA. These are
unnnecessary DUPES and not just COPIES I am talking about. They are
just wasted QRM...
>So, if home RELAY fill ins are spaced properly, what is the problem?
>They are doing what they were put there to do...
So true. But is the other 95% of the time that that mobile is
transmitting RELAY,WIDE and hitting the big digis direct that is causing
horrendous duplication all the rest of the time when he *is* in range
of digis that is the problem...
>I fail to understand your statement that home RELAYs
>cause so much duplication..
They don't. What I was meaning was that the "concept of home RELAYS
which is the basis for mobiles using the RELAY,WIDE... path is causing
horrendous duplication most of the time when the mobile hits other
digipeaters directly" is the better way to say it. Its not the home
"RELAYs" but the use of the RELAY,WIDE... path that is the problem.
>So in short, a DUPE to me is a packet sent by a RELAY
>that was received from another RELAY. It is not a DUPE
>if two RELAYs attempt to digipeat the same received
>direct from the tracker's packet.
Yes. A DUPE is the same digi digipeating the same packet more than once...
>Simple solution is to limit unnecessary home RELAYs.
No, Best solution is to eliminate the use of the RELAY,WIDE and WIDE,WIDE
paths completely in the USA. This will cause a huge 2 to 5 TIMES reduction
in dupes. This means the network load could go down to 50% or even 20%
of what it is now. With that kind of a quiet channel there probably is
no need for many of the fill-in digis in the first place, because now
the regular digis might hear enough dead silence to hear the mobile in
what used to be called the dead zone in the first place.
And if the dead zone is really dead, then simply replace the RELAY alias
with the WIDE1-1 alias. It will work just as well, but being of the
WIDEn-N family, the perfect dupe-elimination filter in the digis will not
generate the wasteful dupes. Thus, users that need to use FILL-IN digis
can use the dual entry WIDE1-1,WIDE2-1 path. Notice the second entry
starts as 2-1 so that only one additional hop is used for a total of 2
which is about right in most areas. A mobile path of WIDE1-1,WIDE2-2
would make 3 hops on every packet. OK in remote rural areas, but not
desired in the seven major APRS metro areas or California or any other
populous state with big mountain digis.
NEVER use WIDE1-1 anywhere except the first hop in a path. Just like RELAY
was never used beyond the first hop, or it generates horrendous QRM.
Hope that clears it up.
Bob, WB4APR